qcow: check more address addition for overflow
qcow: check more address addition for overflow

Check that two address calculations don't overflow. Return an error if
they do. Thanks cargo fuzz.

Change-Id: I9dd301c4e08516cea8456f1ea313d62582979da0
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/932646
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm